In general, the recording track on the optical disk is scanned by a laser light to effect data recording or reproduction while, as the optical disk is rotationally driven at a constant angular velocity or at a constant linear velocity, a focusing servo or a tracking servo is applied to an optical head having enclosed therein a laser diode driven by a laser driving circuit to output the laser light for data recording or reproduction, a photodetector for detecting the reflected laser light from the optical disk, or the like.
On the other hand, similarly to the so-called sector servo for a magnetic disk, a so-called sampled servo technique has been proposed, according to which a recording track in the form of concentric circles or a helix on the optical disk is spatially divided into a servo area and a recording area, tracking data pits or the like are previously formed or preformated in the servo area as servo signals at a predetermined interval or angle, and in which these discrete servo signals are sample-held when the disk is rotationally driven to perform a continuous servo control.
In addition, with the recording/reproducing apparatus for an optical disk in which the recording track is scanned by the laser light to perform data recording and reproduction, if the light beam power is too low during recording, data cannot be recorded reliably, whereas, if the light beam power is too high during reproduction, data recorded on the recording track may be destroyed or otherwise affected seriously. Thus, as disclosed for example in the Japanese Patent Publication No. 53-46633 (1988), it has been customary to detect the volume of the laser light for data recording and reproduction and to switch a so-called automatic power control (referred to hereinafter as APC), servo loop, which performs a feedback control of a driving circuit of a laser diode outputting the laser light to maintain a constant light beam power, as a function of the operating modes, to make a proper switching of the laser beam power.
On the other hand, the recording/reproducing apparatus for an optical disk may exhibit different characteristics, such as reflectivity or modulation factors, depending on the type of the recording medium, that is, the optical disk, such that the input signal to a servo system, such as the focusing servo or tracking servo system, may vary from one optical disk to another, so that desired servo characteristics may not be obtained. Hence, with the conventional recording/reproducing apparatus for an optical disk, as shown in FIG. 1, the playback RF signals detected by an optical head 50 are supplied from a preamplifier 51 to a servo circuit 53 via variable gain amplifier 52 so that desired servo characteristics will be obtained by the servo circuit 53 through automatic gain control or so-called AGC of the variable gain amplifier 52.
In the conventional recording/reproducing apparatus for an optical disk, shown in FIG. 1, a spindle servo is applied to an optical disk 54 so that the disk is rotationally driven by a spindle motor 55 so that the disk is rotated at, for example, a constant angular velocity. The optical head 50 also has enclosed therein a laser diode which is driven by a laser driving circuit 56 to output a laser light for data recording or reproduction, a photodetector for detecting the reflected laser high from the optical disk 54, and so forth. The light volume of the laser light output from the laser diode is feedback-controlled by an APC circuit 57 so that the output light volume of the laser diode will be constant and, with the focusing servo and the tracking servo being applied by the servo circuit 53, the recording track on the optical disk 54 is scanned by the laser light to perform data recording/reproduction.
The detection output of the photodetector detecting the reflected laser light from the optical disk, that is the RF playback signals detected by the optical head 50, is supplied via preamplifier 51 to the variable gain amplifier 52 and from the variable gain amplifier 52 to the servo circuit 53 and to a clock regenerating circuit 60, while being supplied via sample hold circuit 61 to an automatic gain control circuit 62.
The clock regenerating circuit 60 extracts clock components contained in the playback RF signals supplied via variable gain amplifier 52 by so-called PLL to perform clock detection to produce various clocks necessary to perform system control.
The sample-hold circuit 61 processes the playback RF signals, supplied via variable gain amplifier 52, by sample-holding modulated outputs A and B by tracking data pits Q.sub.A and Q.sub.B previously recorded in a servo area SA of a recording track TK of the optical disk 54, and an unmodulated output C, obtained between the tracking data pit Q.sub.B and the clock data pit Q.sub.C, and by supplying the outputs A to C to the automatic gain control circuit 62.
The automatic gain control circuit 62 processes the playbackRF signals output via variable gain amplifier 52 by controlling the gain of the variable gain amplifier 52 on the basis of the sample-held outputs A to C by the sample-hold circuit 62 so that the sum of the modulation factors by the tracking data pits Q.sub.A and Q.sub.B will be constant. Meanwhile, write pulses consistent with record data are supplied to the laser drive circuit 60 during the record mode operation from a recording block 59.
Meanwhile, in order to perform automatic gain control (so-called AGC) of the detection output by the optical head 50, as in the case of the conventional recording/reproducing apparatus for an optical disk shown in FIG. 1, it is required of the variable gain amplifier 52 to have frequency characteristics of extremely broad bandwidth, such as 0 to about 6 MHz, as well as satisfactory characteristics, such as DC offset, drift or S/N ratio. The variable gain amplifier satisfying these various properties are usually costly and bulky in size so that they are not suited to an ultra-small size optical disk system employing a 5- or 3- inch size optical disk.
In view of the above described status of the art, it is an object of the present invention to provide a recording/reproducing apparatus for an optical disk in which a recording track on which a data record area and a servo area having tracking data pits formed therein are formed in spatial isolation from each other is scanned by a laser light to record or reproduce data on or from the recording track, and which is inexpensive and simple in structure. Thus the present invention provides a recording/reproducing apparatus for an optical disk in which servo signals detected from the servo area may be maintained at a predetermined signal level by a simplified circuit construction.